1. Field of the Invention
The invention relates to a transmission system employing a transmitter including a splitter for splitting up a transmission signal into a signal having a low frequency range and a signal having a high frequency range, and a first coding device for deriving a coded signal having a low frequency range from the signal having a low frequency range. The first coding device is arranged for transmitting the coded signal having a low frequency range to a receiver by a first transmission channel. The receiver employs a first decoder for forming a reconstructed signal having a low frequency range based on the coded signal having a low frequency range. The transmitter further employs a second coding device for deriving a coded signal having a high frequency range from the signal having a high frequency range, which second coding device is arranged for transmitting the coded signal having a high frequency range from the transmitter to the receiver by a second transmission channel. The receiver further employs a second decoder for forming a reconstructed signal having a high frequency range based on the coded signal having a high frequency range by using a noise signal coming from a noise signal source, and a combiner for combining the reconstructed signal having a low frequency range and the reconstructed signal having a high frequency range.
The invention further relates to a transmitter, a receiver, a coding device, a decoder, a coding method and a decoding method to be used in a transmission system of such type.
2. Description of the Relation Art
A prior art transmission system is known from EP 0 648 024 A1. This document describes a transmission system for audio signals in which the input signal is split up into spectral portions by a filter bank. These spectral portions are coded each by its own coding device, a sub-coder. In the sub-coder, the envelope of a signal is determined and this envelope is compared with a number of reference envelopes. An identification code of the reference envelope corresponding best to the envelope is transmitted to a receiver.
In the receiver, a decoder reconstructs a signal on the basis of the identification code, the envelope of which signal corresponds to the received reference envelope, after which the envelope is multiplied by a noise signal coming from a noise source, which results in a reconstructed spectral portion of the input signal. Subsequently, these reconstructed spectral portions are combined to thus form a reconstruction of the input signal.
A disadvantage of such a transmission system is that the coding device needs to have considerable computation capacity for the splitting up of the input signal into spectral portions by a filter bank in the transmitter and the combining of the spectral portions in the receiver with the aid of a combiner.
It is an object of the invention to provide a transmission system in which the necessary computation capacity is reduced.
For this purpose, the transmission system according to the invention is characterized in that a coding device comprises analysis means for determining prediction coefficients and for transmitting the prediction coefficients to a receiver and in that a decoding device is arranged for filtering the noise signal coming from the noise signal source during the reconstruction of the signal having a high frequency range by means of an LPC synthesis filter which is controlled by the prediction coefficients.
The input signal is split up into two portions, so that an optimum coding for each of the two frequency ranges can be selected. A first coding device utilizes a known coding, which is efficient for a signal having a low frequency range, at an associated efficient bit rate. A low-pass filler is sufficient for this signal. A second coding device utilizes the Linear Predicive Coding (LPC) to code the signal having a high frequency range in an efficient manner. Thanks to the properties of the LPC coding, a high-pass filter is sufficient and it is not necessary to apply down-sampling. Since the high-pass filter and the low-pass filter both require little computation capacity, and a down-sampler is omitted, the total required computation capacity is reduced.
The human auditory system in this high frequency range is considerably less precise, so that it is possible during the reconstruction of the signal having a high frequency range, to take a white noise source as a signal source and subsequently fitter the signal source with an LPC synthesis filter, so that a signal is obtained whose spectrum to the human auditory system sufficiently matches the original signal. Since it is avoided that the high frequency range is subdivided into smaller frequency ranges, which are to be processed separately, the required computation capacity is reduced.
An embodiment of the transmission system according to the invention is characterized in that the second coding device in the transmitter is arranged for generating an amplification code based on the signal having a high frequency range and in that the second decoder in the receiver is arranged for utilizing the amplification codes during the reconstruction of the signal having a high frequency range.
Since the coding device determines an amplification code by which the decoder subsequently amplifies the reconstructed signal, the number of prediction coefficients required is reduced, so that determining the prediction coefficients becomes simpler and requires less computation capacity.
The frequency ranges can be determined.